This invention relates to a printing press registry system, particularly for a cylinder press such as a screen printing press.
Cylinder presses such as those employing stencil screen printing require the stock that is to be printed is first position-registered prior to the print stroke. Typically, automatic side registry systems previously used on presses have involved a three point registry system to automatically register the stock prior to printing. When printing sheet stock, two permanent guides were typically located on the printing cylinder to achieve proper front-to-rear register of the sheet to the cylinder/stencil screen. The third registry point was on the side of the sheet to achieve proper side-to-side registry. This side registry was done automatically with either a push guide or a pull guide that moves laterally. A push guide is the simplest method of side registry, but is restricted to sheets stiff enough to allow pushing without buckling. Many materials are too thin or flexible to push, and therefore need to be pulled. Over the years several systems to do this have been employed. Each system had its benefits but also significant problems.
During the 1950s, several pull type registry devices used a pull guide formed of a slow moving wheel located just below the surface of the feed board. A pivoting pressure wheel was located above the sheet stock. At the appropriate time, the upper wheel would be moved down by a cam, putting pressure on the sheet and the lower rotating wheel. This pressure would cause the lower wheel to propel the sheet laterally to a fixed stop block. Problems with this design included the fact that it was difficult to adjust for various sheet widths, difficult to adjust to set the right pressure necessary for pulling various types of sheet stock, and the fact that it could not be moved from one side of the press to the other e.g., from the operator side to the nonoperator side of the press, to perform registry on the opposite side. It was not practical to move it mechanically from one side to the other.
During the 1960s and 1970s, registry devices such as that in U.S. Pat. No. 3,120,180 were used. This involved a vacuum pull guide which became a standard for over 20 years. The brass slide that was used for the push guide was changed to include a vacuum chamber and manifold. The brass slides were mechanically moved laterally a fixed distance in and out by a cam, or moved pneumatically. The vacuum was turned on and off at a predetermined point in the cycle, controlled by a cam and a four way valve. The sheet was first transported linearly up the feed board to the temporary stop which prevented further linear advancement, the vacuum would then be activated and the brass slide moved laterally, carrying the sheet laterally to a fixed stop block. Since the slide had to travel a fixed distance, and the amount of side registry travel required for the sheet would vary based upon its delivery position from the feeder, the operator had to regulate the vacuum suction on the sheet very precisely. This was so that, when the sheet hit the stop block, the vacuum seal to the sheet would break and release the sheet while the slide continued to its fixed position. If there was too much vacuum, the sheet would buckle when it hit the stop. If there was too little vacuum, the sheet would slip and not consistently hit the stop. Further, it was difficult to regulate the amount of vacuum required for accurate registry at various press speeds. It was also difficult to change from a push type to a pull type guide, or visa versa. Further, if the stock was wavy it was difficult to suck it down to the brass slide and hold it while moving the sheet into registry. Additional problems with this design included considerable consumption of time to change it from one side of the press to the other side.
Consequently, in the 1980s, presses in the U.S. tended to employ an imported feeder and side registry system from Germany. In this type of device, the register slides were moved mechanically by a cam located on the feeder. It would push or pull the sheet into registry. The pull guide consisted of a moving gripper finger which would grab the sheet and pull it to a fixed stop block. Tension of the gripping finger i.e. the amount of clamping force on the sheet, was controlled by a spring. The tripping/releasing action of the gripper finger was controlled by two pins located on the fixed stop block. Problems with this design included the fact that the operator had to adjust the cam on the feeder every time he wanted to change from push type to pull type registry, and this was difficult. Also, there were too many mechanical parts which wore out rapidly. Further, to pull heavy stock, the pressure needed from the spring to grip the sheet caused the parts to wear very rapidly, requiring frequent repairs. Finally, it was difficult to set the stop block and pull finger relationship.
A prior type apparatus used on presses for web stock employed a vacuum platen registry system controlled by a linear actuator and photosensitive eye. This worked well on presses printing web stock where the amount of pull to registry was only one-eight inch or less, but such does not respond rapidly enough for a cylinder press printing sheet stock. Until presently therefore, various ones of these above-described registry systems have been employed on sheet printing presses. All of these take considerable time to complete the registry.
Some presses are understood to use a vacuum pull guide in conjunction with a photoelectric eye sensor, although it is not known by the inventors herein how the sensor activates or controls the vacuum platen.
There is often a need to reliably move sheet stock up to one-half inch for registry, and a requirement to do so in a fraction of a second.